Automatic control of fluid catalytic cracking units

ABSTRACT

A fluidized catalytic cracking unit is controlled for carbon balance and maximization of a secondary control function by using a general purpose digital computer, which is responsive to various temperatures and pressures, as the controlling means for variables such as the settings on the regenerator flue gas control valve and the regenerated catalyst circulation control valve. The process of the present invention involves obtaining a plurality of signals representing control variables, comparing these with the desired values for the control variables, and carrying out in a general purpose computer the calculation of a carbon adjustment factor according to the algorithm: WHERE EACH OF THE VARIABLES HAS THE DEFINITION SET FORTH IN THE SPECIFICATION. When Delta SVn is positive, indicating a correction for carbonburning conditions, the desired control action is obtained by a logic sequence which determines whether either of the controlled variables is under constraint and if not, takes the control step most consistent with optimization of the secondary variable. If one or both of the controlled variables are under constraint, the logic sequence indicates the correct control step to be taken or, if no control step can be taken, indicates that none can be taken. When Delta SVn is negative, indicating a correction for carbonbuilding conditions, the logic sequence likewise allows the choice of the optimum control step to be taken or, if none can be taken, indicates this fact. After the logic sequence based upon the algorithm has been completed, a signal is obtained to move the correct control valve (that is, make the required adjustment in the controlled variable), the signal being corrected to reflect the difference in control function (depending upon which valve is to be moved) and for the position of the valve immediately prior to movement to the new position. By carrying out the control function of the present invention, carbon balance can be well controlled in a catalytic cracking unit while the secondary control variable (such as regenerator air velocity) can be controlled or maximized.

United States Patent [72] inventor Robert E. Zumwalt Blytown, Tex.

[21] Appl. No, 801,388

[22] Filed Feb. 24, 1969 [45] Patented July 6, 1971 [73] Assignee EssoResearch and Engineering Company [54] AUTOMATIC CONTROL OF FLUIDCATALYTIC CRACKING UNITS 7 Claims, 2 Drawing Figs.

[52] U.S.C1. .......235/151.12, 208/164 [51] Int. Cl ..G05b 15/02, ClOg13/14 [50] Field of Search 208/164; 235/15 1.12

[56] References Cited UNITED STATES PATENTS 3,175,968 3/1965 Berger208/164 3,261,777 7/1966 Iscoletal.... 208/113 3,378,483 4/1968 Worrellet a1 208/164 3,410,793 11/1968 Stranahan et al.. 208/164 PrimaryExaminer- Eugene G. Botz Attorneys-Thomas B. McCulloch, Melvin F.Fincke, John S.

Schneider, Sylvester W. Brock, .lr., Kurt S. Myers and Timothy L.Burgess ABSTRACT: A fluidized catalytic cracking unit is controlled forcarbon balance and maximization of a secondary control function by usinga general purpose digital computer, which is responsive to varioustemperatures and pressures, as the controlling means for variables suchas the settings on the regenerator flue gas control valve and theregenerated catalyst circulation control valve. The process of thepresent invention involves obtaining a plurality of signals representingcontrol variables, comparing these with the desired values for thecontrol variables, and carrying out in a general purpose computer thecalculation of a carbon adjustment factor according to the algorithm:

IF r n)+ z]( n)+ n[ where each of the variables has the definition setforth in the specification,

When ASV is positive, indicating a correction for carbonburningconditions, the desired control action is obtained by a logic sequencewhich determines whether either of the controlled variables is underconstraint and if not, takes the control step most consistent withoptimization of the secondary variable. If one or both of the controlledvariables are under constraint, the logic sequence indicates the correctcontrol step to be taken or, if no control step can be taken, indicatesthat none can be taken.

When ASV, is negative, indicating a correction for carbonbuildingconditions, the logic sequence likewise allows the choice of the optimumcontrol step to be taken or, if none can be taken, indicates this fact.

After the logic sequence based upon the algorithm has been completed, asignal is obtained to move the correct control valve (that is, make therequired adjustment in the controlled variable), the signal beingcorrected to reflect the difference in control function (depending uponwhich valve is to be moved) and for the position of the valveimmediately prior to movement to the new position.

By carrying out the control function of the present invention, carbonbalance can be well controlled in a catalytic cracking unit while thesecondary control variable (such as regenerator air velocity) can becontrolled or maximized.

IIJIISCHON CARIOI uuntmm 5'2 cuuuu oultumi 524 522 *5" attosneer QEE'EZ"ulcer re sv r CIIICI IIAIU or autumn vumuits, RCZVAVJFQHA All vnoun,near I wuss llll VFCUCII I SELCII resv 556 PATENIED JUL 6|97I 3;,591'783SHEET 1 [IF 2 --I--FLUE GAS T I F.G. SLIDE VALVE US I US I REGENERATOR Ir I 2|2' A PRODUCT VAPOR I I I (9- I I I I r I08 I I I "1 I I I I I I II I 203 CARBON BALANCE I CONTROLLER I I 300 (COMPUTER) I REACTOR I I 302300 I I 2 I l I I 205 I I PRIMARY SECONDARY: a FG/REG AIRVELOCITY: I I IA OR REACTOR I I TEMP. I /l I I20 I OPERATOR-SET I M AIO I TARGETS I I22I -'-lO6 I I l R.c.s.v. I I l FIG I I ||4-\ I I I04 I l l 20 207 =lI2 II I 209% FEED AIR I I I I L. I

INVIJN'! ()l\.

ROBERT E. ZUMWALT,

ATTORNEY.

PATENIED JUL 6 IHII VAR PR 500 s TACK AT CALC. CURRENT R T N VALUE OFENTRY STACK AT 502 CALC. SLOPE EL E '3 sAvE TIME N sAvE STACK AT 506 v RP R USE A A 1?- CALC. PENDING DELTA T CARBON BALANCE RTN E ADJ FROMBASIC E'i EL EL ALGORITHM USE s PECIAL coEFF's.

CHECK sTATus OF CARBON BURNING BELOW TARGET TARGET AT OR AT 0R BELOWABOVE TARGET A E 2 AIR I\-/( "I VELOCITY RESTRAINT VARIABLES; RCSV AP,REGEN. AIR

VELOCITY, REGENT mass.

DIRECTION CARBON BUILDING AT OR ABOVE TARGET aEl.ow

TARGET SELECT R c s v AT OR BELOW TARGET TARGET ADJUST V R c sv SELECT554 F G sv :55;; AT OR ABOVE TA RGET RTN SELECT M 5 F G s v 556 N 0ACTION ADJUST I TAKEN F G s V INVEN'I'UR. RTN RTN ROB ERT E ZUMWA LT,

A T TOR NE Y.

AUTOMATIC CONTROL OF FLUID CATALYTIC CRACKING UNITS The presentinvention is directed to a control procedure whereby a secondary controlobjective (such as regenerator air velocity) can be controlled ormaximized at the same time that the carbon balance of a fluidizedcatalytic cracking unit is being kept under control. Basically, thepresent invention is devoted to the use of a general purpose digitalcomputer (such as the IBM 1800) to carry out the periodic sampling of aplurality of variables (such as temperatures, pressures, and flowrates), compare certain of these values with predetermined limits, useother of the sensed values in solving the algorithm for carbon balancecontrol, perform a logic sequence to select the proper control step tobe taken (e.g., choosing between the flue gas slide valve and theregenerated catalyst slide valve), generate a corrected signalproportional to the amount of desired change in the controlled variable,and transmit such signal to the controlled valve to accomplish thedesired correction. The entire sequence is repeated at short intervalsof time (e.g., l-3 minutes apart).

The invention can best be understood by the following jdescription,wherein the catalytic cracking unit will be described, as well as theequipment used in the cracking unit for generating control signalsproportional to the operating variables and the equipment used,responsive to signals from the computer, to accomplish the directedchanges in the operating procedure.

Referring now to FIG. 1, a catalytic cracking unit is schematicallyshown as comprising a reactor I and a regenerator 102. It is to beunderstood that the reactor may be a dense phase fluidized bed reactoror a disperse phase transfer-line reactor; either is suitable insofar asthe present control function is concerned. The feed to the reactor isintroduced by way of a line 104 and admixed with hot, regeneratedcatalyst which is conducted by way of line 106 from the regenerator 102.

Within the reactor 100, the feedstock is catalytically cracked, yieldinga product vapor of lower average molecular weight than the feed. Thisproduct vapor is removed by way of vapor line 108 for fractionation andfurther treatment. A byproduct of this cracking reaction is coke, whichcontains hydrogen and carbon. This coke is deposited on the catalystparticles and tends to deactivate the catalyst. The coke is burned fromthe catalyst particles by contact with air in the regenerator section ofthe cracking unit. It is the balancing of coke laydown and burnoff rateswhich provides the present invention with its great utility.

The catalyst from the reactor, after separation from the product vapor,is removed by way ofline 110 and is conducted to the regenerator 102.The catalyst may be directed to the regenerator by gravity alone if thereactor is located higher than the regenerator or it may be lifted intothe regenerator by at least a portion of the regenerator air supplywhich is introduced by way ofline 112. The remainder ofthe regeneratorair is schematically shown in FIG. 1 as being introduced by way ofa line114.

Within the regenerator 102, the coke which has been laid down on thecatalyst particles during the reactions in reactor 100 is contacted withoxygen in the air and burned to produce water vapor, carbon dioxide andcarbon monoxide. The water vapor and gases are removed by way of lineI16 through the flue gas slide valve (FGSV) 118.

Since the coke produced in the reactor is approximately 90 weightpercent carbon, it is commonly referred to as carbon" and it will be socalled hereinafter.

Regenerated catalyst is discharged from the regenerator by way of line120, and is passed through the regenerated catalyst slide valve (RCSV)I22 and thence through line 106 for introduction into the reactor ashereinabove/described.

It is imperative that carbon balance be maintained in operating acatalytic cracking unit; that is, the amount (rate) of carbon burned inthe regenerator must equal the amount of carbon produced in the reactor.If more carbon is produced than is burned, carbon will build up on thecatalyst to such an extent that the reaction is severely hindered, theresult being a diminution in conversion and an increase in thepercentage of undesirable products being produced. On the other hand, ifmore carbon is burned than is produced, carbon will be burned off thecatalyst and the resulting excess (unconsumed) oxygen in the flue gaswill react with carbon monoxide in the phenomenon called after-burning."The afterburning reaction generates heat and, if it becomes too severe,will create temperatures high enough to damage the regenerator.

The control of carbon balance depends upon the correlation of severalprocess temperatures, from which inferences may be drawn concerningcarbon building or carbon burning situations. A number of suitabletemperature differentials may be used as indicators of carbon balance,such as (l) the difference between flue gas and regenerator bedtemperatures, (2) the difference between the temperature of the catalystentrant into the regenerator and the regenerator bed temperature, or (3)the difference between the temperatures at the top and the bottom of thecatalyst riser line (in those units which .lift the catalyst into theregenerator via air entrainment). The

particular temperature differential to be used will depend upon the typeof unit employed. Generally, the-difference between flue gas andregenerator bed temperatures will be suitable. In the followingdescription it will be assumed that the flue gas-regenerator bedtemperature differential is to be employed as the parameter indicatingcarbon balance conditions.

The difi'erence between flue gas and regenerator bed temperatures iscalled the stack AT. In normal operations, the flue gas temperature willbe from 10 to 50 F. higher than the temperature of the dense bed, due tothe afterburning reaction between a slight amount of excess oxygen andcarbon monoxide in the flue gas. When the stack AT increases, it isindicative of an increase in the amount of excess oxygen which isavailable for reaction with the carbon monoxide in the flue gas.

Under these conditions, the air rate is too high for the then-existingcarbon production rate (or, stated in another way, the carbon productionrate is too low for the then-existing air rate), and the unit is said tobe operating under "carbonburning conditions (i.e., the net percentageof carbon left on the regenerated catalyst is being reduced).

Similarly, when the stack AT decreases it is indicative of a deficiencyof air at the then-existing carbon production rate and the unit is saidto be operating under carbon-building" conditions (i.e., the netpercentage of carbon left on the regenerated catalyst is beingincreased).

The carbon balance of the unit is a function of catalyst circulationrate, air rate, feed rate, catalyst holdup in the reactor and feedquality. Generally, however, the carbon balance control function iscarried out quite simply by adjusting either the flue gas slide valve(FGSV), which affects air rate and catalyst circulation rate, or theregenerated catalyst slide valve (RCSV), which affects only the catalystcirculation rate, or both. As used hereinafter, when it is stated that avalve is to be opened" or closed, it is to be understood to mean anincremental change in slide valve position in the direction of fullyopen or fully closed, respectively. During normal operation of the unit,neither the FGSV or the RCSV may be fully closed, and only rarely arethey (or either of them) fully open.

There are some unit restraints which make it undesirable to makeadjustments in one or both of the slide valves under certain conditions.For example, the AP across the RCSV must be maintained at or above agiven minimum value in order to avoid the danger of reverse circulationwhich might bring hydrocarbon feedstock into the regenerator where itwould contact the regeneration air. Further, the regenerator pressuremust operate between certain maximum and minimum limits.-

increased. It can, however, be partly closed in response to a controlsignal, since the A? would thereby be increased. Further, theregenerator pressure cannot be reduced when the RCSV AP is at or belowits desired minimum, since this would itself have the effect of loweringthe AP across the RCSV. Therefore, under those conditions the FGSVcannot cannot be further opened, but can be moved toward the fullyclosed position in response to a signal. Likewise, when the regeneratorpressure is at or below its desired minimum, the FGSV cannot be furtheropened, since this would have the effect of reducing regeneratorpressure; and, when the regenerator pressure is at or above the desiredmaximum, the FGSV cannot be further closed, since this would have theeffect of increasing regenerator pressure.

All of these factors are taken into consideration in the presentinvention.

The control elements which are used in the present invention arebasically well known. The temperatures which must be determined aresensed by thermocouples and a signal directly related to the temperatureis generated and sensed by the computer as a part of the periodicsampling of variables. The pressure sensors likewise generate a signaldirectly related to the pressure, which is available to the computerwhen it samples. The pressure drop AP across the RCSV is sensed by awhat is commonly referred to as DP cell" (standing for differentialpressure cell") and a signal directly related to that pressure drop isgenerated and made available to the computer upon demand. The rate offlow of air supplied to the regenerator is obtained either by an orificemeter located in a combined air line or by individual orifice meters ineach of the air source lines. In the latter case the total air rate isobtained by summation of the individual rates. Likewise, a signaldirectly related to the rate of flow is made available to the computerupon demand. The thermocouples which are used typically made from ironand constantan thermocouple wires and may be constructed by the user orpurchased from a manufacturer. The DP cell suitably may be a FoxboroModel l3A-l. The pressure sensor and recorder may be a Foxboro 61 lGM-ASZ. The orifice meter may be a Foxboro M/6l3DM.

FIG. 1 represents a suitable system, but it is to be understood thatother sensors may be used instead of those shown therein. Referringagain to FIG. 1, the control system is seen to comprise a temperaturesensing element 201 which is located in the flue gas line near theregenerator. Although it is shown for clarity as being locateddownstream of the FGSV, it can as well be located between theregenerator and the FGSV and preferably would be so located. A similarsensing means 203 is supplied for sensing the bed temperature within theregenerator. It is located below the upper level of the fluidized bed ofcatalyst within the regenerator, so as to sense the dense phase bedtemperature rather than the temperature in the disperse phase. Atemperature sensing means 205 may be likewise provided in the reactor100, if the reactor temperature is to be used as a secondary controlvariable. Usually, however, the secondary control variable will be theregenerator air velocity, calculated from the amount of air supplied tothe regenerator, e.g., as sensed by the orifice meter 207, which inpractice may be a plurality of orifice meters located in each of aplurality of ducts feeding air into the regenerator and the computeradds the resultant signals to obtain total in flow.

in the scheme shown in FIG. 1, the regenerator air velocity is used asthe secondary control objective as indicated by open valve 208 andclosed valve 209. However, where the reactor temperature is to be usedas a secondary control objective, it will be substituted for the signalfrom the orifice meter. Although valves are shown as controlling thesignals from the secondary control sensing points, implying the use ofpneumatic signals, it is to be understood that electronic signals mayand preferably will be used in supplying information to the computer300. The regenerator pressure is sensed by means of pressure gauge 212,and the AP across the RCSV is sensed by the DP cell 214. Thus, it isseen that signals representing each of the material variables aresupplied and made available to the computer.

The computer is programmed to sample each of the material variables on arepetitive basis. The period of time between cycles of sampling willhereinafter be referred to as the sample period. Upon reading each ofthe material variables for a given sample period, the computer is thenable to carry out the control function and, if required, produce signalsfor repositioning the FGSV and/or the RCSV, These signals would betransmitted by way of lines 300 and 302. Control signals could,preferably, be provided as a number of pulses which would be sensed byan indexing motor which adjusts the valve position controller, which inturn controls the motor which drives the slide valves. A Foxboro 67 HTGMmotor valve is suitable for such use. Other systems, such as a DDC-typeoutput station, can be used In some such cases the indexing motor wouldnot be required.

ALGORITHM.

The control upon which the primary carbon balance control is based isstated as:

where AS V is the change in slide valve position for the nth samplingperiod expressed in percent based upon slide valve travel, from fullyclosed to fully open-being percent;

K, is the reset control constant;

K, is a control constant greater than zero;

(ST, is the deviation at the nth sampling period, defined as the stackAT at the nth sampling period minus the desired value of stack AT;

Abs(6T,,) is the absolute value of 8T,,;

K, is the proportional control constant;

AT, is the temperature difference represented by said first signal atthe nth sampling period;

AT,, is the temperature difference represented by said first signal atthe nthl sampling period; and

Al is the period of time between samplings.

The first part of the algorithm:

r is proportional to the reset gain constant in standard parlance,although the inclusion of the deviation 8T as a part of the expressionmakes the value variable with the deviation. The use of a positive valuefor K, makes the total value of the reset constant a significant amountwhen the deviation approaches zero, so that a control function can beobtained. The use of the absolute value of the deviation as a part ofthis expression allows the sign or deviation, 87),, by which theexpression K,[Abs(6T,,)-l-K,]is multiplied, to control the sign or ofthe total value of the reset portion of the algorithm.

The deviation in the expression is the difference between the actualstack AT and the desired stack AT. It is obvious that the larger thedeviation from the desired target, the greater the control functionshould be. This is accomplished in the al gorithm by including thedeviation 8T, twice, in essence making the square of the deviation apart of the control adjustment ASV,,. Of equal importance is the rateand direction in which the deviation is changing. If the deviation isincreasing in a positive direction, the control action should recognizethe fact that it is moving in the positive direction. If, however, thedeviation is decreasing, then the control function should recognize thefact that a smaller control correction is required. The second portionof the algorithm A T A T,, At

Thus, it is seen that in the algorithm, the first term is equivalent tothe error squared reset mode of control, wherein good control can bemade on the basic task of maintaining carbon balance. A sample period(i.e., the interval of time between initiation of two successivesamplings) of 2 minutes has been found to be about the best sampleperiod for which good control response can be obtained on the particularunit on which the control system was tested,l and generally sampleperiods of l-3 minutes will be suitable for most units. In no eventshould the sample period exceed'30 minutes. For best results, the tuningconstants K,, K, and K, must be chosen so that the slope termpredominates. A high rate of change in the stack AT requires thatcorrective action be taken, regardless of whether stack AT is above orbelow its target.

The change in slide value position in response to a repositioningcommand must be limited in order to avoid upset of the unit. The limitASV,,,,,,. must be chosen carefully, because of the predominance of theslope term in the algorithm. When the stack AT is changing rapidly, theslope term may indicate a desired" change of several percent for theslide valve. Such a large change is not desirable, since it may lead toupset conditions in the unit. In effect, by setting a maximum limit onASV, a limit is set on the gain of the control loop under conditions oflarge carbon imbalance. This leads to much smoother and better control.For most installations, ASV should be about 1 percent to 2 percent ofslide valve travel, depending on whether the RCSV or the FGSV is usedfor control. The FGSV can tolerate a higher ASV than the RCSV.

Solution of the algorithm gives both the direction of control and theamount of control that needs to be applied. The sign of ASV,, indicateswhether correction for carbon building or carbon burning should be made;a positivevalue indicates correction for carbon burning and a negativevalue indicated correction for carbon building. Assuming that acorrection for carbon burning is to be accomplished, the RCSV may beopened the required amount or the FGSV may be closed the requiredamount. Where neither slide valve is under restraint, the proper controlvalve will be chosen in view of the secondary control variable. Forexample, if the air velocity is at or above target, and the regeneratorpressure is not at or above the desired maximum, the FGSV will be closedrather than opening the RCSV. However, if the air velocity is belowtarget, the RCSV will be opened, thereby reducing regenerator pressureand allowing a greater flow of air into the regenerator. This selectionis done by the computer based upon a logic sequence introduced by theprogram.

When air blower capacity is not limiting the production rate of thecracking unit, the velocity is usually controlled very closely to itstarget, thereby stabilizing the operation of the unit. When blowercapacity is limiting, the use of a target velocity high enough to beunachievable will cause the computer program to open the FGSV until theRCSV minimum AP constraint is reached. The air rate to the unit isthereby maximized, and is consistent with the carbon balance. Bymaximizing air rate, unit capacity is maximized.

THE COMPUTER.

A general purpose computer, such as the IBM 1800 and the GE 4020, may beused for the entire control sequence, beginning with the sampling of thedata points and ending with the generation of control signals for theoperation of the slide valves. The computer may be programmed by usingany suitable machine language or programming language, such as GPCP[discussed in Generalized Process Control Programming System" by Ewinget al. published in Vol. 63 No. l of Chemical Engineering Progress Jan.1967), pages 104- 1 l0], PROSPRO IBM Application Program H-026l-0),Standard Fortran language or basic machine language.

The operation of the computer in response to the program is shown inFIG. 2, which is a block diagram of the carbon balance control sequence.Referring now to FIG. 2, it is seen that the first action of thecomputer is shown in block 500 as calculating the current valve of stackAT, which is accomplished by sampling the flue gas temperature (e.g.,from sensor 201 in FIG. 1) and the regenerator bed temperature (e.g.,from sensor 203 in FIG. 1). The calculated stack AT is then retained.The second action shown in block 502 is to calculate the slope or rateof change of stack AT. This is done by comparing the stack AT obtainedin Operation 1 with the stack AT from the previous sampling period. Theslope is then calculated as previously discussed. The new stack AT isretained as well as the calculated slope.

The third operation is the basic control function. If external means arenot being used to control excessive afterburning (such as theintroduction of torch oil into the regenerator to consume excess oxygen)the carbon balance adjustment from the basic algorithm can be computed.When means such as torch oil are being used, special coefficients(larger reset control constant and zero proportional control constant)must be used to prevent the resulting rapid decrease in stack AT fromcausing an erroneous control action to be taken (i.e., a correction forspurious carbon-building conditions). The additional oxygen consumptionin the regenerator is due to externally supplied hydrocarbons ratherthan carbon laid down on the catalyst, and is itself a corrective stepfor the carbon-burning conditions that actually exist.

The computer than begins the third operation by receiving a signal todetennine whether means such as torch oil are being used, as indicatedin diamond504, and when it is not, it utilizes normal coefficients tocalculate ASV, as indicated in block 506. If the thus calculated 166 8V,is not a significant adjustment as determined in diamond 508 [e.g., 0.08percent (one step on stepping motor) or greater], no operation iscarried out at the end of that sampling period and this adjustment valueis retained. If a significant adjustment is called for; the computer (asshown in block 510) samples the RCSV AP, regenerator air velocity, andregenerator pressure to obtain the current values thereof to determinewhether the unit is operating under any constraint. As shown in diamond512, the computer then selects the logic path to be followed, dependingupon the sign of ASV,,. If ASV, is positive, indicating a carbon-burningcorrection, it follows the sequence of first comparing the RCSV AP withits lower limit, as represented by diamond 514, and if the RCSV AP isabove the lower limit previously set, then the secondary controlvariable is examined. As shown in diamond 516, the air velocity signalis compared with the desired value and if it is below the target value,a signal is obtained that the RCSV is to beadjusted as shown in block518. If the air velocity is at or above target, however, the regeneratorpressure is then compared to the predetermined maximum regeneratorpressure. If the regenerator pressure is at or above the predeterminedmaximum, a signal is obtained to adjust the RCSV, thereby helping carbonbalance but not air velocity. If the regenerator pressure is below thepredetermined maximum, a signal is obtained to adjust the FGSV, as shownin block 522, thereby helping both carbon balance and air velocity.

If, however, in the first step it was determined that the RCSV AP was ator below the lower limit, the pressure signal from the regenerator iscompared to the predetermined maximum pressure as shown in diamond 524,and if the regenerator pressure is below the maximum limit, a signal isobtained to adjust the FGSV, thereby helping carbon balance, but not airvelocity. If the regenerator pressure is at or above the predeterminedmaximum pressure, the entire unit is in constraint and no action can betaken (as indicated in block 526) and neither carbon balance nor airvelocity can be helped.

On the other hand, if the sign of ASV,, (in block 512) is negative,indicating carbon-building conditions, the first step in the logicsequence is to sample the secondary control variable, air velocity, asshown in diamond 550. If the air velocity is at or above the target, asignal is obtained to adjust the RCSV (block 551), thus helping carbonbalance but not air velocity. If the air velocity is below target, thenthe RCSV AP is compared to its minimum limit, and if the RCSV AP is ator below the minimum, then a signal is obtained to adjust the RCSV(block 551), again helping carbon balance but not air velocity. If theRCSV AP is determined as shown in block 552 to be above thepredetermined minimum, then the regenerator pressure is compared to thepredetermined minimum pressure and if it is at or below that minimum,then a signal is obtained to adjust the RCSV (block 551). If in the steprepresented by diamond 554 the regenerator pressure is above thepredetermined minimum pressure, then a signal is obtained to adjust theFGSV as shown in block 556, thus helping both carbon balance and airvelocity. it is thus seen, by advertence to boxes 518 and 551 thatsignals to adjust the RCSV can be obtained and, by advertence to boxes522 and 556, it is seen that control signals to adjust the FGSV can beobtained. Note that when FGSV control is possible, both carbon balanceand air velocity are helped.

Each of these control signals must be modified to correct for thepeculiarities of control by utilizing the FGSV as compared to the RCSV.The modifications are discussed below.

The control signal for adjustment of the RCSV results from amodification of ASV, (calculated in block 506) and is expressed by thefollowing equation:

ARCS l byxK dA HS V, where ASV, is the output from the carbon balancealgorithm (block 506),

K is proportional to the overall gain of the algorithm,ex-

cept when ARCSV is limited to the specified quantity ARCSV Thecorrection factor K is generally obtained by trial and error tuning onthe process. The parameters K, and ARCSV. have been found to vary withslide valve position. They are assumed to be linear functions of theslide valve position and are changed over the limits of the valve travelfrom about 23 percent to about 45 percent. From to about 23 percentopen, a minimum value is established; and from 45 percent to 100 percentopen, a maximum value for each is used. in operation, this method ofvarying the gain has proved to be suitable over a wide range ofoperating conditions.

The adjustment equation for the FGSV is expressed by the equation:

Note that the output of the carbon balance algorithm is still used, ASV,but the correction factor has the opposite sign. This is due to the factthat the flue gas slide valve works in the opposite direction from theregenerated catalyst slide valve in accomplishing the same controlobjective. The gain coefficient K was estimated to be unity, and workedsatisfactorily when the slide valve position was not close to its limitsof operation, that is, fully open or fully closed. The maximumadjustment per pass, AFGSV,,,,, was chosen as 1 percent; i.e., slidevalve position can be changed at each adjustment no more than 1 percentof the distance. from fully open to fully closed.

In a fluidized catalytic cracking unit wherein the present controlsystem has. been successfully employed, the following values have beenfound to be successful:

a. In the algorithm:

K,=0.00l2 percent per F.

K,= F. K,,=0.375 percent per F.minute AFZ minutes (sampling period) b.Correction factor: K,,=0.033(VP)-0.333

c. Slide valve change limits: ARCSV,,,,,,=0.0267( VP)O.267 percentAFGSV,,,,,,=1.0 percent These values were used when indicating carbonbalance by the stack AT (i.e., flue gas temperature bed temperature inthe regenerator). In K and ARCSl/ the factor (VP) is the position of theRCSV expressed as percent open, and must be within the range from 23percent to 45 percent.

Having disclosed my invention in detail, what is to be protected byLetters Patent is to be determined by the appended claims and not be thespecific examples and disclosures hereinabove given.

I claim:

1. A method for controlling carbon balance in a fluid catalytic crackingunit while concurrently maximizing a secondary control variable whichcomprises A. supplying to a digital computer a plurality of signalsrepresenting control variables:

a first signal representing a temperature differential between twolocations in the unit, chosen to provide an indication of carbon balancein the unit,

a second signal representing the pressure drop across the regeneratedcatalyst slide valve,

a third signal representing the regenerator pressure, and

a fourth signal representing a secondary control variable;

B. supplying said digital computer with values representingpredetermined limits for the variables represented by said second andthird signals, with the predetermined target value of the temperaturedifferential represented by said first signal, and with thepredetermined target value for said secondary control variable;

C. when the regenerator is not being supplied with torch oil carryingout in said digital computer the following sequence of steps, repeatedlyat predetermined sample periods not greater than 30 minutes,

l. determining the carbon adjustment factor, ASV,,, ac-

cording to the algorithm:

where ASV, is the indicated correction in slide valve position for thenth sampling expressed in percent based upon slide valve travel, fromfully closed to fully open being percent;

K, is the reset control constant;

K, is a control constant greater than zero;

8T, is the deviation at the nth sampling;

K is the proportional control constant;

AT, is the temperature difference represented by said first signal atthe nth sampling;

AT is the temperature difference represented by said first signal at thenthl sampling; and

At is the sample period between samplings;

2. when ASV, is positive, indicating a correction for carhon-burningconditions, determining the desirable control action by the logicsequence:

a. if the pressure drop across the regenerated catalyst slide valve isabove the predetermined minimum limit therefor and the secondary controlvariable is below its target value, obtain a signal to reposition theregenerated catalyst slide valve;

b. if the pressure drop across the regenerated catalyst slide valve isabove the minimum limit therefor and the secondary control variable isat or above its target value and the regenerator pressure is at or abovethe predetermined maximum limit therefor, obtain a signal to repositionthe regenerated catalyst slide valve;

0. if the pressure drop across the regenerated catalyst slide valve isabove the minimum limit therefor and the secondary control variable isat or above its target value and the regenerator pressure is below thepredetermined maximum light therefor, obtain a signal to reposition theflue gas slide valve;

d. if the pressure drop across the regenerated catalyst slide valve isat or below the minimum limit therefor,

and if the regenerator pressure is at or above the predetermined maximumlimit therefor, take no control action, but if the regenerator pressureis below the predetermined maximum limit therefor, obtain a signal toreposition the flue gas slide valve;

. when AS V,, is negative, indicating a correction for carbon-buildingconditions determining the desirable control action by the logicsequence:

a. if the secondary control variable is at or above its target value,obtain a signal to reposition the regenerated catalyst slide valve;

b. if the secondaryc'ontrol variable is below its target value and thepressure drop across the regenerated catalyst slide valve is at or belowthe predetermined minimum value therefor, obtain a signal to repositionthe regenerated catalyst slide valve;

c. if the secondary control variable is below its target value and thepressure drop across the regenerated catalyst slide valve is above thepredetermined minimum limit therefor, and the regenerator pressure is ator below the predetermined minimum limit therefor, obtain a signal toreposition the regenerated catalyst slide valve;

d. if the secondary control variable is below its target value and thepressure drop across the regenerated catalyst slide valve is above thepredetermined minimum limit therefor, and the regenerator pressure isabove the predetermined minimum therefor, obtain a signal to repositionthe flue gas slide valve;

4. and repositioning the valve for which a repositioning signal isobtained.

2. A method as in claim 1 further comprising the steps:

a. if the selected valve is the regenerated catalyst slide valve,

correcting said signal by the equation:

ARCS 1",, K A SV.,

where ARCSV is the corrected amount of movement to be made in theregenerated catalyst slide valve, no greater than ARCS V,,.,,

K is the regenerated catalyst slide valve correction factor,

ASV,, is the carbon adjustment factor, and

ARCSV is from I to 2 percent of slide valve travel,

to obtain a repositioning signal proportional to the desired amount anddirection by which the regenerated catalyst slide valve is to berepositioned;

b. if the selected valve is the flue gas slide valve, correcting saidsignal by the equation:

where AF GS V, is the corrected amount of movement to be made in theflue gas slide valve, no greater than AF GS V" K is the flue gas slidevalve correction factor, and

AFGSV is from 1 to 2 percent of slide valve travel,

to obtain a repositioning signal proportional to the desired amount anddirection by which the flue gas slide valve is to be repositioned; and

c. repositioning the selected valve in response to the respectiverepositioning signal, opening said valve when the signal is positive andclosing said valve when the signal is negative.

3. A method in accordance with claim 1 wherein the secondary variable isregenerator air velocity.

4. A method in accordance with claim 1 wherein the secondary variable isreactor temperature.

5. A method in accordance with claim 1 wherein the sample period isabout 2 minutes.

6. A method in accordance with claim 1 wherein the secondary variable isregenerator air velocity, the regenerator air velocity is below itspredetermined target value, and the sampling period if about 2 minutes,whereby air rate to the regenerator is maximized.

7. A method in accordance with claim 1 wherein, when torch oil isintroduced into the regenerator, K, is increased by a factor of two tofour times and K ,,=O.

1. A method for controlling carbon balance in a fluid catalytic crackingunit while concurrently maximizing a secondary control variable whichcomprises A. supplying to a digital computer a plurality of signalsrepresenting control variables: a first signal representing atemperature differential between two locations in the unit, chosen toprovide an indication of carbon balance in the unit, a second signalrepresenting the pressure drop across the regenerated catalyst slidevalve, a third signal representing the regenerator pressure, and afourth signal representing a secondary control variable; B. supplyingsaid digital computer with values representing predetermined limits forthe variables represented by said second and third signals, with thepredetermined target value of the temperature differential representedby said first signal, and with the predetermined target value for saidsecondary control variable; C. when the regenerator is not beingsupplied with torch oil carrying out in said digital computer thefollowing sequence of steps, repeatedly at predetermined sample periodsnot greater than 30 minutes,
 1. determining the carbon adjustmentfactor, Delta SVn, according to the algorithm: where Delta SVn is theindicated correction in slide valve position for the nth samplingexpressed in percent based upon slide valve travel, from fully closed tofully open being 100 percent; Kr is the reset control constant; Ks is acontrol constant greater than zero; delta Tn is the deviation at the nthsampling; Kp is the proportional control constant; Delta Tn is thetemperature difference represented by said first signal at the nthsampling; Delta Tn 1 is the temperature difference represented by saidfirst signal at the nth-1 sampling; and Delta t is the sample periodbetween samplings;
 2. when Delta SVn is positive, indicating acorrection for carbon-burning conditions, determining the desirablecontrol action by the logic sequence: a. if the pressure drop across theregenerated catalyst slide valve is above the predetermined minimumlimit therefor and the secondary control variable is below its targetvalue, obtain a signal to reposition the regenerated catalyst slidevalve; b. if the pressure drop across the regenerated catalyst slidevalve is above the minimum limit therefor and the secondary controlvariable is at or above its target value and the regenerator pressure isat or above the predetermined maximum limit therefor, obtain a signal toreposition the regenerated catalyst slide valve; c. if the pressure dropacross the regenerated catalyst slide valve is above the minimum limittherefor and the secondary control variable is at or above its targetvalue and the regenerator pressure is below the predetermined maximumlight therefor, obtain a signal to reposition the flue gas slide valve;d. if the pressure drop across the regenerated catalyst slide valve isat or below the minimum limit therefor, and if the regenerator pressureis at or above the predetermined maximum limit therefor, take no controlaction, but if the regenerator pressure is below the predeterminedmaximum limit therefor, obtain a signal to reposition the flue gas slidevalve;
 3. when Delta SVn is negative, indicating a correction forcarbon-building conditions determining the desirable control action bythe logic sequence: a. if the secondary control variable is at or aboveits target value, obtain a signal to reposition the regenerated catalystslide valve; b. if the secondary control variable is below its targetvalue and the pressure drop across the regenerated catalyst slide valveis at or below the predetermined minimum value therefor, obtain a signalto reposition the regenerated catalyst slide valve; c. if the secondarycontrol variable is below its target value and the pressure drop acrossthe regenerated catalyst slide valve is above the predetermined minimumlimit therefor, and the regenerator pressure is at or below thepredetermined minimum limit therefor, obtain a signal to reposition theregenerated catalyst slide valve; d. if the secondary control variableis below its target value and the pressure drop across the regeneratedcatalyst slide valve is above the predetermined minimum limit therefor,and the regenerator pressure is above the predetermined minimumtherefor, obtain a signal to reposition the flue gas slide valve;
 4. andrepositioning the valve for which a repositioning signal is obtained. 2.when Delta SVn is positive, indicating a correction for carbon-burningconditions, determining the desirable control action by the logicsequence: a. if the pressure drop across the regenerated catalyst slidevalve is above the predetermined minimum limit therefor and thesecondary control variable is below its target value, obtain a signal toreposition the regenerated catalyst slide valve; b. if the pressure dropacross the regenerated catalyst slide valve is above the minimum limittherefor and the secondary control variable is at or above its targetvalue and the regenerator pressure is at or above the predeterminedmaximum limit therefor, obtain a signal to reposition the regeneratedcatalyst slide valve; c. if the pressure drop across the regeneratedcatalyst slide valve is above the minimum limit therefor and thesecondary control variable is at or above its target value and theregenerator pressure is below the predetermined maximum light therefor,obtain a signal to reposition the flue gas slide valve; d. if thepressure drop across the regenerated catalyst slide valve is at or belowthe minimum limit therefor, and if the regenerator pressure is at orabove the predetermined maximum limit therefor, take no control action,but if the regenerator pressure is below the predetermined maximum limittherefor, obtain a signal to reposition the flue gas slide valve;
 2. Amethod as in claim 1 further comprising the steps: a. if the selectedvalve is the regenerated catalyst slide valve, correcting said signal bythe equation: Delta RCSVn Krc Delta SVn where Delta RCSVn is thecorrected amount of movement to be made in the regenerated catalystslide valve, no greater than Delta RCSVmax Krc is the regeneratedcatalyst slide valve correction factor, Delta SVn is the carbonadjustment factor, and Delta RCSVmax is from 1 to 2 percent of slidevalve travel, to obtain a repositioning signal proportional to thedesired amount and direction by which the regenerated catalyst slidevalve is to be repositioned; b. if the selected valve is the flue gasslide valve, correcting said signal by the equation: Delta FGSVn -KfgDelta SVn where Delta FGSVn is the corrected amount of movement to bemade in the flue gas slide valve, no greater than Delta FGSVmax, Kfg isthe flue gas slide valve correction factor, and Delta FGSVmax is from 1to 2 percent of slide valve travel, to obtain a repositioning signalproportional to the desired amount and direction by which the flue gasslide valve is to be repositioned; and c. repositioning the selectedvalve in response to the respective repositioning signal, opening saidvalve when the signal is positive and closing said valve when the signalis negative.
 3. when Delta SVn is negative, indicating a correction forcarbon-building conditions determining the desirable control action bythe logic sequence: a. if the secondary control variable is at or aboveits target value, obtain a signal to reposition the regenerated catalystslide valve; b. if the secondary control variable is below its targetvalue and the pressure drop across the regenerated catalyst slide valveis at or below the predetermined minimum value therefor, obtain a signalto reposition the regenerated catalyst slide valve; c. if the secondarycontrol variable is below its target value and the pressure drop acrossthe regenerated catalyst slide valve is above the predetermined minimumlimit therefor, and the regenerator pressure is at or below thepredetermined minimum limit therefor, obtain a signal to reposition theregenerated catalyst slide valve; d. if the secondary control variableis below its target value and the pressure drop across the regeneratedcatalyst slide valve is above the predetermined minimum limit therefor,and the regenerator pressure is above the predetermined minimumtherefor, obtain a signal to reposition the flue gas slide valve;
 3. Amethod in accordance with claim 1 wherein the secondary variable isregenerator air velocity.
 4. and repositioning the valve for which arepositioning signal is obtained.
 4. A method in acCordance with claim 1wherein the secondary variable is reactor temperature.
 5. A method inaccordance with claim 1 wherein the sample period is about 2 minutes. 6.A method in accordance with claim 1 wherein the secondary variable isregenerator air velocity, the regenerator air velocity is below itspredetermined target value, and the sampling period if about 2 minutes,whereby air rate to the regenerator is maximized.
 7. A method inaccordance with claim 1 wherein, when torch oil is introduced into theregenerator, Kr is increased by a factor of two to four times and Kp 0.